Freecad has a famously bad boolean solver, but creating temporary booleans is essential to split models up for printing. When it fails, it normally leaves off a face. Cura can't slice it unless it's a closed solid.
Cura shows a goofy pattern where a boolean error has happened & it slices it into garbage. Fortunately, you have a friend in the boolean business: Blender. Just go to file->import to import the STL file.
Blender shows 2 faces are missing. It's just a matter of going into edit mode, hitting C to select the vertices around the holes & hitting F to create faces.
Then, export it as an STL file. Cura is happy to load the STL file generated from blender, with all the right dimensions.
Booleans in Freecad are a pain in the mane anyways, with goofy offsets & goofy tweeks normally required. Blender can import the STL files for the model & print mask.
Then apply the boolean by "adding a modifier" to the object to be printed & hiding the print mask object. Of course, we still need a tiny offset to get rid of some dangling faces. At least blender shows the dangling faces instead of completely failing.
Blender converts 1mm to 1 meter. .1 meter in Blender is .1mm.
You now have to export the STL file with "apply modifiers" enabled.
Cura slices booleans from Blender just as well as Freecad.